Chilly air billows out from the entrance of an abandoned mine on a steep hillside, providing a refuge for Ice Age fauna and flora.

Every winter, the wind carves blocks of frozen water into beautiful abstract shapes inside this unusually curved tunnel.

The largest wildlife crossing in Utah allows local fauna to safely traverse the six-lane Interstate 80 at Parleys Canyon.

This geothermal hot spring hides beneath a slowly growing mineral dome.

The only ski tunnel in North America provides easy access to amazing snow and beautiful views.

This Roman underground canal was the longest tunnel ever built until the late 19th-century.

In a pedestrian tunnel on North Carolina State’s campus resides an expressive and artistic free-for-all.

Nine caves filled with recently-discovered archaeological finds under the houses and the streets of the most ancient district of an old town.

An eighth-century rock-cut cave temple in the middle of Pune.

Spectacular mineral formations fill these caves, which have been used for everything from smoking meat to stashing illicit liquor.

An underground wonderland in Iowa.

The highest point in Alabama was almost completely deforested, but is now home to protected towers and trees.

Since it formed millions of years ago, this cave system has been a sacred burial ground, a Prohibition-era speakeasy, and the center of an amusement park.

Behind this beachfront shop lies a spectacular, colorful sea cave with an equally colorful past.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

A Mars simulation in the southern Utah desert.

Bizarre lunar-like landscape with thousands of large stone "hoodoos."

This roadside stop between Goblin Valley is the perfect location to view some of the region's natural wonders.

At this gas station in the middle of the desert, a convenience store has been carved directly into the rock.

The largest underground lake in Europe.

Out of a labyrinth of abandoned tunnels and stops, this Queens subway station is arguably the most mysterious—if it exists at all.

The crypt under this cemetery's chapel was used as a meeting place by Filipino revolutionaries.

A luxurious doomsday bunker hidden beneath an unassuming Las Vegas home offers the best of subterranean suburbia.

An extravagant western hotel where every floor is the "ground floor."